soil test and consult with your local ag extension/soil & water district for input.
not knowing your situation and conditions (and seeing it) it is hard to give the best advice.
some generalities:
spring seeding - highest competition from existing weeds, etc., best growing conditions
fall seeding - little to no competition, decent growing conditions
frost seeding - for more northern climates, spread seed when there is frost/feezing at night and warmer temps during the day